Method of forming films from cellulose derivatives



Oct. 28, 1941.

METHOD OF FRMING FILMS FROM CELLULOSE DERIVATIVES G. A. DALlNj 2,260,741

Filed oct.v 1o, 195e;

2 Sheets-Sheet l WASH FORM IOct. 28, 1941. G. A. DALIN 2,260,741

METHOD OF FORMING FILMS FROM CELLULOSE DERIVATIVES Filed Oct. 10, 1938 l2 Shees-Shee't 2 l f RY Patented Get. 28, 1941 Unirse vv2,260,741;h Y `Mefrnon or roar/[ING FILMS FROM CELLU- i s s Losenemvia'rlvs.v

`George A. Dalinyfilartford, Conn., assignery to Flax Corporation, "Hartford, Conn., a corporation of Delawarev l Appiieanon october 1o,

y1i claims'.

d This invention relates to methods of forming lms such as hollow articles or sheets from solutions of organic compounds, particularly solutions of cellulose derivatives such-as cellulose acetate, with suitable plasticizer, inracetone or other vvolatile solvent. i

-It has been proposed prior to the presentinvention to formv hollow articles by dipping forms in a solution of cellulose acetate. This is laccomplished by immersing glass or metal forms in ythe solution to form films thereon which films are dried by evaporation of the solvent and are then stripped from the forms.` The dipping, forms may be mounted in framesor carriers of a Amachine so that a large number of films orarticles can be formed in each cycle of operations.

Althoughuseful articles of various kindsmay be produced by prior dipping methods, such methods 'have certain disadvantages ywhich adversely affect the quality of the products and the cost of producing them. One disadvantage is that a relatively dilute solution of cellulose acetate in acetone must be employed. Satisfactory articles may be produced from' a dilute solution by a single dippingv operation without difculty, but such lms are very thin, being not in excess of about five-thousandths (.005) of an inchr in thickness whereas films of fifteenfto twenty-thousandths (.015 Vto .020) of an inch usually are desired. Therefore, the forms must be'dipped in `the solution 'a number of times to build up films of the desired thickness. Usually, at least three dips are necessary, each dip'being followed by a drying period. l

However, increasing the number of dipping operations tends. to` increase the formation of b'ub'- l bles in the films. This tendency might be reduced by drying the films in an atmosphere ofv Likewise in casting films in sheet form from a l' solutions of cellulose derivatives dilute solutions are employed in prior methods in order to minimize the formation of bubbles in the sheets and tozfacilitate casting. Thesolution is casto-n a wheel or band from a hopper which has a doctor y.

193s, serial Ne'.-`2,3'4,o9,7

(cris-57) tionable because of'thelimitation `on the thickness ofv sheetewhich `can'be -formed in this man# ner, the longV time required for drying and because of .the fcost-` of-solvent employed. To ob'- tain thicker,v sheets; cast lms have to be laminated orV the sheets have to be cut from a pressed cake, or block of-materiaL f The general object of the presentinvention is to provideajnovel method of formingfilms, such as `hollow articles orjsheets, from solutions of cellulose derivatives which methodreduces or eliminates the'above and other disadvantages pf prior methods.V l

, A more specific object of the invention is to provider anovel method, of forming lms' or i articles from solutions. of, cellulose derivatives by the use of. vwhich `the-tendency to formf bubbles in the lms or articles isrnaterially reduced.

Another object is to provide a novel method v by theemplo'yment l`of which materially thicker lms ofcellulose derivati-ves `may beeobtlained lfrom solutions thereof. f bya ,singlerdipping or casting operation .than lin :priorimethodai j Aiurther object'oftheinvention is to provide a rvnovel `methodof;forming, films from solutions `ofcelli'ilose derivatives by which thelms may be prepared morerapidlyftha-ny in prior methods. .....Other objects .and advantageswill bepointed 4out in, ,or will` become .apparent-from, the detailed description which follows and the l accompanying 'drawings which depictthe lpreferred procedure embodying. .thefnovel method randfin f 0method involves thepreparation of a solution "by dissolving cellulose acetate v.and suitable plasticizerznia relatively small amount of acetone to providea relatively viscous solution. V- This solu- -blade associated therewith for regulating the -thickness of the sheet; The sheet is dried by evaporation of the solvent and when suiciently dry is4 stripped from the casting surface. .The use of dilute solutions for casting also is objec- ,the invention and,

which drawings'zj", n Fig.. 1 is a, diagram` offthe` sequence of `steps 1n forminglms lay-dipping in accordance' with Fig.' -2 isa diagram `oftl'ie sequence of `steps which may be followed in casting sheets when employing my invention.

Considered in its broader aspects, lmy'novel tion is made by stirringwhil'eheating to av temperature just below boiling', say 51 C., and per-k jmittinggthe solventfv'apors formed to ypass into a reflux condenser. f f

When the celluloseacetate and plasticizery are .Completely-dissclvedinthe acetone, the' s'o1uy.tien is treated to-remove all or substantially all 4'dissolved gases or vapors and particularly air letoilewhich arepassecl intoafreiiux condenser.

Preferably, the heating or boiling is effected at atmospheric pressure although it may be accomplished under vacuum. As a result of this treatment dissolved gases and vapors are swept out by the acetone vapors as they are driven off from the solution and pass out of the system through the reiiux condenser. This greatly reduces the formation of bubbles in the films formed. from the solution when the films are dried, If the viscosity of the solution is too high at this time, it may be adjusted to the desired value by the addition of boiled acetone. An excess of solvent may be added to the initial mixture to compensate forA loss thereof caused by boiling or heating. The solution is now ready for the dipping or casting operation.

In the production of films by dipping (see Fig. 1) suitable forms are 4clipped into the above s0- lution to form films thereon. The forms may be made of metal or glass and preferably are first washed and then may be dipped in a suitable liquid to providel an undercoating which will act as a lubricant and assist in stripping the films from the forms. This 'undercoating may be dried before the forms are dipped in the dipping solution. Examples of undercoating materials are soap solution, molten or dissolved wax, or a solution in water of glycerine, gelatine or water soluble gum and sugar. If an undercoatingv is not employed, the forms should be highly polished.

The dipping solution preferably is maintained at a temperature just below boiling, say 51 C. while the forms are dipped therein. By maintaining the solution at such temperature, its viscosity is low enough to permit the dipping operation to be performed. In other words, the solution which I employ is too viscous for the forming of films at ordinary or room temperature. However, at a high enough temperature, say 51 C., the viscosity is low enough so that by using a proper rate of withdrawal of the form from the dipping solution a film can be prepared which is of suitable thickness, andv does not have a tail suspended from the lower end` of the form. However, if it is desirable to form films under such conditions that tails form or tend to form as, for instance, where films of increased thickness are desired, such tails may be avoided by withdrawing the forms at a sharp angle, say 45 degrees or less, to the surface of the solution while slowly revolving them. Tails can also be eliminated by very quickly flipping the forms to cause the tails to fall over on the bodies of the films which permits the tails to be absorbed by the films before the tails and the films set.

The films are now dried at about 45' C. while the forms are rotated horizontally to prevent the films from running.

During this drying operation, wrinkles form or tend to form in the films. This is due to the fact that evaporation of the acetone proceeds from the outer surface, which hardens and assumes a definite size and shape. When the inside portion of the film begins to harden it shrinks and causes or` tends to cause wrinkles to form on the outside of the film. I have discovered that this wrinkling can be prevented by treating the films with a solvent vapor. purpose the Vapor of a solvent which has a higher boiling point than acetone and ethyl acetate is preferred although other solvents may be used with good results.

The films are treated with ethyl acetate :at intervals during the drying period, for example, by holding them in the vapor of the solvent provided I prefer to use for this by heating a quantity of the ethyl acetate in a jacketed container at a temperature of about 61 C The ethyl acetate vapor condenses uniformly on the films and softens their surfaces thus removing wrinkles or incipient wrinkles.

The speed at which the films dry is not appreciably reduced by this treatment because only a small quantity of ethyl acetate is absorbed. However, the effect of the ethyl acetate lasts for a few minutes because of its comparatively low vapor pressure and therefore it does not have to be renewed frequently. Ordinarily, the films are treated with ethyl acetate in the above manner three times at intervals in the drying ofthe films at about` 45 C After such treatments, the films are free of wrinkles and can be further dried without wrinkling.

Further drying of the films may now be effected until drying is complete or nearly so. The nal drying may be accelerated by increasing the temperature in stages in increasing periods of time.

When sufficiently dried, the lms are stripped from the forms. If the forms have been treated with an undercoating material, the films are soaked in water to soften such material. The water for soaking may be heated. The films also may be soaked, for a much longer period, if no undercoating material is employed. If highly polished forms are used, that is, chromium or nickel plated forms, or forms of polished brass, soaking of the films may be omitted. In this event tapered forms are employed.

The films are stripped from the forms by applying a jet of air beneath the upper edge of each film. After removal from the forms the films are washed and dried. The forms are now washed and dipped in the undercoating material, if such material is employed, and the undercoating dried. rlhe forms are now ready for another cycle of film forming operations.

EXAMPLE 1.--Dippi1zg A composition for carrying out the invention may be prepared .according to the following for- The cellulose acetate employed preferably has a viscosity of 4 seconds as determined by the viscosity test of the A. S. T. M.

The plasticizer may be any suitable composir tion such as methyl phthalyl ethyl glycollate.

Other types of plasticizers may be employed singly or in combination.

The above composition is characterized by the relatively small amount of acetone which it contains. Dipping solutions heretofore employed usually have contained about four pounds of acetone to one pound of cellulose acetate. The above composition and method permit a reduction of approximately 40 per cent in the amount of acetone used. This results in substantial savings in the cost of 'the dipping solution and hence of the finished products.

The above materials in approximately the proportions set forth are mixed by stirring and heating at about 51 C. in a jacketed vessel connected with a reflux condenser. When solution is completed, it is boiled for about 5 minutes and then cooled to 51 C. The viscosity is adjusted to the desired value by the addition of boiled acetone.

Viscosity 'oil the. solution may' be determined by ascertaining the time required' for' a' oneeighth inch steel ball to fallbetween two circlesL twol and? one-half inchesv apart: inscribed around atwo quart jar containing the solution at 51Cl y Y Preferably the viscosity as above determined is adjusted to between-7.0 and 11.0 secondsy at 51 C., the method' being carried outvmostsuc.- cessfully in this range. Itwill be observed that this range of viscosity is materially higherthan the vviscosity of dipping.l solutions heretofore used which vary froml about l@ toA I second as determinedfbythe" above method. i

The; dipping solutionv prepared as aboveA explained, is` held in a suitable vessell and maintainedl at atemperature ofy approximately 51? C., that is; at a temperaturejustbelowy the boil.- ing point, while the previously prepared forms are dipped' therein. Y I

The forms may be prepared by washingii'n water for about 2 minutes; drying and dipping them in an undercoating'solution ofi approximately the following: composition:

Grams v Gelatine= 100 Dextrosef. Y .L H 100 Glycerine 20` Water 1 800 The above solutionis heated at about 45 Cl.

The forms are now dried at about 120 C. for about l minutes to harden the undercoating thereon. n

The. forms are now dipped once onlyyin the dipping solution and withdrawn. The rate vof withdrawal is varied. according to the viscosity of the` solution and the-l thickness of' the film desired, .the thickness. increasing` with the rate of. Withdrawal. With formsv one inch in diameter and. six inches in length and solutions varying in viscosity from 'fito 11 seconds, rates of `Withdrawal varying fromY aboutA .1'7"/sec.; to

2"/sec. will. produce. films varying in thickness from aboutv .006" to about. .025".Vv For examplel with. a solution'having a. viscosity of 8.6. seconds, and a rateY of withdrawalof. about .27' inch` per secondffllms of approximately .020' inch inV thickness will be formed. With a sufiiciently viscous solution. and, a slow rateoi withdrawal, lms up to fty-thousandths (1050).. .ofy anr inchmay be formed by a single dipping operation.

The forms` are now subjected,V to a temperature ofabout. 45 C. and rotated in horizontal positions to dry the films. After dryingfor about 2. minutes, the: forms. are.l held in ethyl acetate vapor for about 2 secondsagainv dried, at 45 C. for 5. minutes, placed in ethyl.. acetate vapor for 5 seconds, dried at 45 C..for Y9.4 minutes and. finally held in. ethyl acetate vapor for `1.0 seconds. 'I'he films are now dried at 45 C. for l0 minutes, at 55 C. for 15 minutes and 65 C. for 69 minutes.

Preparatory to stripping lthezjifilmsfrom the forms., they'aresoaked in water at 5-1 C..-for 50 minutes. This softensl the undercoating and permits the films to be" blown oi the forms by an air jet.

I The lms may now be washed in wateru and dried.

Numerous advantages result from ymy novel method. Chief among these is the formation of a relatively thick nlm by a' single dipping operation*- This is made possible by the treatment of the solution to remove dissolved gases therefrom which in turn permits the use of a sufficientlyl concentratedsolution to form a thick fl'mby a singledipping: operation which film A 'and is heated' to about 45 C., a given point on the y is 1 free of bubbles which. would otherwise be formed from such gases. The production of such thick films' vis also made possible by the rtreatment of the fllmswith ethyl acetate which prevents Wrinkling` and reduces strain` in the films.`

Another advantage is Athe saving of acetone previously mentioned which reduces material costs and drying time. Dryingy time isV principally reduced, however, by drying at high temprature. The use of high temperature may reb ducethedrying time of prior methods up to. fty per cent.

Still another advantage is the reduction in mechanism and mechanical operations resulting from the use of one instead of three dipping opera-tions.

Variations may be made in the composition of the vdipping solution and in the details of the method without departing from the scope of the appended claims. For example, the dipping forms may be heated internally by a heated fluid medium or electrical resistances to assist inv drying the films. This serves to heat the inner'. sides of the films, thus causing `more rapid migration of the acetone outwardly through the iilms and more rapid and more uniform drying than is obtained by heating the films only externally. d

A feature of the method not described above and whi'ch'constitutes a part of the invention 4is the treatment of the dipping solution in the to the pool without being cooled to below the boiling point.

' ExAMrLE 2.-Casting In casting sheetsv according to the method of my' inventlon (see Fig. 2) a solution is prepared 1n substantially the same manner as for dipping and is supplied to a suitable device or hopperr for discharging the solution on the casting surface. The thickness of the sheet is regulated by a doctor blade in known manner. Preferably the solution is kept entirely closed to the point at Whichv it is cast.

The casting surface may be a plate or a revolving wheel but preferably an endless metallic band isA employed. The casting surface need not be provided with an un'dercoating.

The cast lm or sheet is now alternately dried and subjected to ethyl acetate vapor as it is carried along by the endless band. Tov accomfplish this', heating and vapor chambers of suitlable construction are provided in the path of the band and' overlying its sheet bearing sur- The chambers are the proper length with4 face. relation to they speed of the band to provide the desired periods of heatingI and vapor treatment. Thus the sheet passes under a heating chamber sheet moving from one end of the chamber in say 2 minutes. The sheet now passes through the vapor chamber, a pointv thereon moving through the chamberin about 2 seconds.

Yscribed above.

In like manner, the sheet is heated atk 45C. and vapor-'treated for 5 minutes and 5 seconds respectivelykand again .heated vand vapor-,treated for 9 minutes and 10 seconds respectively, Drying is now accelerated by movement of the sheet through heating chambers .toheatthe sheet at 45 C. for 10 minutes, at 55 C. for 15 minutes and at 65 C. for 69 minutes The sheetmay be stripped from the band beyond the last heating chamber and cut into desired lengths or wound into a roll.

- The band on which the sheet is cast-.may be heated from beneath to a temperature of approximately 45 C. to increase the rateof evaporation of the solvent from the sheet, If this is done, the drying periods described above may be shortened.

It will be understood that variations may be made in the periods and temperatures of 'drying treatment and in the number and lengths of periods ofvapor treatment.

By employing my novel method, thicker sheets may be cast at less cost and in less time than by prior methods.

The values of viscosity recited in the claims are vthe values determined by the methodde- In the above vdescription of the invention the temperature of about 51C. has been specified as the preferred temperature at which-the cellulose acetate vsolution is to be heated for dipping and casting. This temperature will be slightly` below the boiling point, It is to be understood, however, that the temperature of the solution may be increased advantageously to or above the that is, above 11 seconds as herein determined,

the solutions may be hea'ted well above 51 C. and above the boiling point or points and this may also be done with solutions of 11 seconds and below in viscosity.

Having described myv claim is: i

1. The method of forming films from a viscous solution of cellulose acetate in acetone which comprises coating a surface with said solution to form a film thereon, drying said film under such'conditions that said film tends to wrinkle, and subjecting said film to a solvent for the cellulose acetate of higher boiling' point than acetone to counteract the tendency of said film to wrinkle.

2. The method of forming films from a viscous solution of a cellulose derivative in a solvent of relatively low boiling point which comprises coating a surface with said solution to form a film thereon, and alternately drying said film and subjecting it to a solvent for the cellulose derivative of higher boiling point than the first-named solvent 3. The method of forming hollow articles from invention, what I -a viscous solution of cellulose acetate ina solvent `of substantially-lower boiling point than ethyl acetate which comprises, dipping forms once'only in said solution to form films of desired thickness thereon," drying said films and at intervals during such 'drying subjectingsaid films -to ethyl acetate to prevent the formation of wrinkles in said films, and vstripping said. filmsr from said forms, said films being finallyfdried on saidforms at increasing temperature in increasing periods of time.

5. The method of forming hollow-articles from a viscous solution of cellulose acetatein a solvent of substantially `lower boiling 4pointvthan.- ethyl acetate which comprises, dripping forms once only in said solution to form films of; desired thickness thereon, `drying I said- -films at -approximately 45 C. and at intervalsduring such drying subjecting said'films to ethyl acetate Vapor to prevent wrinkles in said films, drying said films at increasing temperatures in increasing periods of time, soaking said filma-and stripping said films from said forms. n; w 6. The method of forming relatively thick films of cellulose acetate which comprises coating a surface with a viscous solution of cellulose acetate in a solvent of substantially lower boiling point than ethylacetate to form a film thereon, and alternatelyv drying and subjecting the to ethyl acetate `vapor to uprevent wrinkling `of saidfilm during the drying thereof. '7. The method of forming relatively thick bubble free filmsof cellulose vacetate whichcomprises preparing a viscous solution of cellulose acetate and plasticizer in acetone, boiling said solution `with reflux to` remove dissolved gases therefrom, ypassing said solution into a poo l,'.dipping forms 'forrn a .film thereon,v drying said lm so rapidly as to form wrinkles'therein, and removing said wrinkles. before said film is completelydried by exposing the surface of said' film tothe action of the vapor of a solvent for, the cellulose acetate having a higher boiling point thanacetone.v

9. The method of forming films from a viscous solution of .cellulose acetate in acetone which comprises coating a surface withsaid solution to form a relatively thick film thereon, which tends `to wrinkle when drying, dryingsaid film, andvuniformly condensing on the surface of said film the vapor of a solvent for the cellulose acetate of higher boiling point than acetone toprevent permanent wrinkling of saidfilm Vduring the drying thereof.

l0. The method of forming films from a viscous solution of cellulose acetate in acetone which comprises coating a surface with said solution to form 'a film thereon,I drying said film for approximately two minutes at approximately 45 C., exposing said film to ethyl acetate 'Y vapor for about two seconds, drying said film for approximately five minutes at approximately 45 C., eX- posing said film to ethyl acetate vapor for-approximately five seconds, drying said film for about nine minutes at approximately45iC., exposing said film to ethyl acetate vapor for about 10 seconds, and finally drying said film.

11. 'I he method of forming-films 4from a viscous solution of cellulose aeetate in acetone which comprises coating a surface with said solution to form a lm thereon, drying said film for approximately two minutes at approximately 45 C., exposing said film to ethyl acetate vapor for about two seconds, drying said lm for approximately ve minutes at approximately 45 C., eX- posing said film to ethyl acetate vapor for approximately ve seconds, drying said lm for about nine minutes at -approximately 45 C., exposing said lm to ethyl acetate Vapor for about ten seconds, and nally drying said film at about 45 C. for about ten minutes,v at about 55 C. for about 15 minutes, and at about 65 C. for about 69 minutes.

GEORGE A. DALIN. 

